Microwave power amplifiers are indispensable components in wireless communications systems and inherently nonlinear. High data rate wireless transmissions are commonly realized by bandwidth-efficient modulation schemes such as QPSK (Quadrature Phase Shift Keying) in WCDMA (Wideband Code-Division Multiple Access) and QAM (Quadrature Amplitude Modulation) in OFDM (Orthogonal Frequency Division Multiplex) in the case of limited bandwidth. The information in these schemes is carried by both amplitude and phase of the RF signal. However, intermodulation distortion is a common problem that results from high-efficiency amplification when power amplifiers are operated close to the saturation region. Spectral regrowth causes both the adjacent channel interference and the degradation of the signal-to-noise ratio. Since there is a stringent requirement on spectrum emission, the output power of the amplifier is commonly backed-off so that the amplifier is operated within the linear region, but this normally results in low-efficiency as most DC power is then dissipated in the form of heat. Power amplifiers are critical components which dominate the power consumption of portable devices during transmission. Therefore, linearity as well as efficiency are highly desirable objectives of power amplifier designs.
In view of these problems various wideband linearization techniques have been explored, in particular feedforward, baseband/second harmonic injection and distortion compensation, which are the three major wideband linearization techniques. Feedforward linearization technique provides an excellent suppression of spectral regrowth over broadband operations, but it requires expensive, bulky and complex circuitry. Baseband/second harmonic injection performs a significant cancellation of the odd-order intermodulation products for a wide frequency range operation. Unfortunately, the improvement is sensitive to the magnitude and the phase of the injected signals, so a complicated control circuitry is implied to guarantee a constant improvement over all the dynamic range of the operating power.
Distortion compensation is a highly cost effective linearization technique and may take both digital and analogue forms. Digital signal processing can provide a precise pre-distortion fitting using accurate look-up tables and high-speed calculations, however it is unusual in handset applications because it requires a large occupation of space on the printed circuit board and also implies additional power consumption. More common in handsets is the use of diode distorters which distort the signal before or after the amplifier. These have the advantages of low cost, simple circuitry and low power consumption. However, the nonlinear characteristic matching between the diode distorter and the power amplifier, as well as the unwanted changes on impedance matching, limit the use of such distorters to specific applications.